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Purely anecdotal (and using the older n Extreme and two AEs), but I never ever got this to work satisfactorily as a base station and two extenders connected over Ethernet: One AE extended the network but with iffy connectivity, the other never worked at all (dotted line sounds familiar).
I think this is another bit of Apple “well, we tried” tech and my Cyberdog browser connected over ISDN through my GeoPort adapter agrees.
Router (a Fritz box) has the WiFi turned off, connected via Ethernet cable to the AirPort Extreme. I configured the Extreme first as a base station, then the Express as an extension of the network created by the base station.
But, when the extreme/express are both operational I lose internet access - turn the express off and the internet comes back.